Changing the starting page with MacOS Big Sure

I have the same issue -

I do not believe people understand that the Bootup Page is not the same as the DESKTOP - WALLPAPER :: So just letting you know I've come to the conclusion that there is no way to CHANGE THE BOOTUP WALLPAPER - just a FYI if anyone decides to update the OS - lol - we would appreciate the ability to change the boot up wallpaper! Thanks!

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and set the picture from there.  Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

That is the answer to your question. There's a YouTube video on hacking Catalina to change the looting screen image. I imagine it will work with Big Sur but have not tried it on either. You won't get a discussion in these forums on how to do it as it's against the terms of use. However, you proceed be sure you have a full and current backup of your boot drive.

I consulted Gary Rosenzweig from MacMost.com, below was his reply.

Yes. This is how it was on Catalina too. The wallpaper is specific to the user account. The login screen is outside the user account -- you aren't logged in yet. A custom wallpaper can't be shown because your Mac doesn't know which user you are yet. And even if it did, you haven't entered your credentials (ID + password) yet to allow access to the settings or custom wallpaper for that account.

Not sure what you mean. Perhaps this: Customize the start page in Safari on Mac - Apple Support

Tell Apple what missing features you'd like restored or new features added to the system via Feedback - macOS - Apple.

Use the System/Desktop & Screen Saver/Desktop preference pane.

You can pick from a number of photos included with the system and any photo you have in your Photos library.

I have the same issue, and it is awful.